X-ray source

ABSTRACT

A compact X-ray source is disclosed, improving controllability and insulation from unwanted high voltage effects. In one aspect, an active variable conductance device ( 130, 330 ) connected in series with the cathode is used in a closed loop, feedback arrangement to control the cathode beam current; the current flowing through the device to the cathode being directly sensed and compared with a desired current level. The result of the comparison is used to control the conductance of the device, thereby directly influencing the cathode current. A second aspect provides an extension of a Faraday cage, whereby the secondary winding of a transformer used to supply power to components within the cage is shielded within a coaxial, tubular member connected to the cage and extending outwardly from it.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a National Phase Patent Application of InternationalApplication Number PCT/GB01/03274, filed on Jul. 23, 2001, which claimspriority of British Patent Application Number 0017976.2, filed Jul. 22,2000.

FIELD OF THE INVENTION

This invention relates generally to the production of X-rays, and inparticular, but not exclusively it relates to a compact X-ray source.

BACKGROUND OF THE INVENTION

A typical X-ray source comprises a thermionic source (typically a heatedfilament), a high-voltage supply to accelerate the electrons to a highenergy, and a target made of a high atomic number metal.

FIG. 1 depicts a simple schematic diagram of a very basic andconventional X-ray source, although it will be realised that, inpractice, much more complex arrangements are generally used, includingthe use of additional electrodes and magnetic fields to control andfocus the electron beam

Electrons are emitted thermionically from a hot cathode filament 30under the action of an isolated heater supply 10 and are attracted to ametal target 70 via an intervening anode 60. The electrons areaccelerated in a beam 50 towards the target due to a high potentialdifference between the filament and the anode/target arrangementestablished by means of a high voltage supply 20. On striking the target70 the electrons stimulate X-ray emission by various processes,resulting in the emission of an X-ray beam 80.

Since it is desirable for the anode and target to be at, orsubstantially near, ground potential, the cathode filament must be at avery high negative potential with respect to ground Moreover, thecathode filament requires several watts of power to reach operabletemperatures.

FIG. 2 shows a typical X-ray source arrangement where a cathode filament30 is heated by a voltage supplied from an isolating transformer 11.Typically the voltage is between 2V and 6V, whilst the electrons areaccelerated by a high voltage supplied from a multiplier 90, known as aCockcroft-Walton voltage multiplier. The high voltage maybe in the rangeof hundreds of kilovolts, for example 160 kV.

It is often required to construct an X-ray source that is compact, andthis requirement introduces or exacerbates various problems, for examplethose associated with providing accurate and effective control over theelectron beam current, particularly where the source is desired to becapable of operating reliably with a low radiation output, and thoseassociated with achieving sufficient insulation between variouscomponents.

Control over the current of the electron beam 50 is usually desirablewith X-ray sources in general and, in low performance X-ray sources,this is frequently achieved merely by varying the temperature of thefilament; relying upon the principle that a hotter filament emits morecurrent than does a cooler one. In higher performance systems,exemplified in very basic form in FIG. 3, this is achieved bycontrolling the beam in the space charge limited regime by means of afield control electrode 40, often referred to as a focusing cup orWehnelt. Such a focusing cup 40 is required to be at a negativepotential with respect to the cathode filament in much the same way asthe grid in a thermionic triode valve. The required potential can besupplied by either an electrically isolated bias supply, or self-biasingusing a feedback resistor 120 between cathode filament 30 and focus cup40. Current passing through the feedback resistor generates the requirednegative bias. However, such a negative feedback system has the drawbackthat it is difficult to adjust.

When conventional X-ray sources are required to operate at low electronbeam current levels, a problem occurs in that electron current leakagefrom the cathode and focus cup becomes significant compared to the totalelectron beam current. Often this problem arises from cold cathodedischarge (field emission), ‘surface tracking’ or other such problematicphenomena. Conventional X-ray sources measure the electron beam currentwith a current sensing circuit located at the end of the high voltagesupply that is at ground potential (shown schematically as 25 in FIG.4). A problem then arises in that any current measurement at this pointin the system cannot differentiate between the actual thermionicelectron beam current and the leakage current. This inability to

separate the level of current leakage from the overall currentmeasurement leads to variations in X-ray output since accurate controlover the true electron beam current is not possible. Particularly wherelow radiation output levels are called for, variations in the measuredelectron beam current due to spurious factors such as those mentionedabove can have a significant and adverse effect upon the radiationoutput levels and stability of operation.

Another problem with conventional X-ray sources arises from the highvoltages required to accelerate the electron beam. When employing suchextreme potential differences, there is always a risk of an electricaldischarge or breakdown. When such phenomena occur, rapidly changingelectromagnetic fields arise. Such fields induce large currents toinstantaneously flow within the electronic circuitry of the X-raysource, and these currents can damage or destroy circuit componentsleading to X-ray source failure. A common solution to this problem is toenclose all susceptible components and circuitry within a Faraday shieldto protect them from any rapidly changing fields.

In known X-ray sources, the integrity of the Faraday shield iscompromised by the need to leave a conduit through which power andsignals can be introduced into the circuitry. The break in the shield toprovide a signal path also provides a pathway for signal interferenceduring a high voltage breakdown. The integrity of the shield isparticularly compromised by the use of isolating transformers that aregenerally used to introduce power and signals into the Faraday shield.

The present invention arose in an attempt to address some or all of theabove problems.

SUMMARY OF THE INVENTION

According to one aspect of the present invention there is provided anX-ray source comprising: a high voltage power source; a cathode filamentcoupled to said high voltage power source; an active variableconductance device connected between the cathode filament and the highvoltage power source; means for determining the amount of currentflowing into said cathode filament through said variable conductancedevice and for providing a signal indicative thereof; and control meansfor utilising said signal to control said amount of current, thereby tocontrol the current of an electron beam emitted from said cathode.

This current control arrangement differs significantly, in concept andeffect, from conventional circuit schemes, which typically employ aseparate DC supply for the grid voltage, floating at cathode potential.The voltage levels of such supplies require accurate control andstabilisation. It has been proposed in U.S. Pat. No. 5,528,657 to usesuch a series-regulating element to control the operative high voltage(anode/cathode) level, but this document does not teach series regulatedcontrol of the grid voltage level. The present invention also differssubstantially, in concept and effect, from circuit arrangements forpulsed grid X-ray tubes, such as those disclosed in Japanese patentapplication No. 59132599. This document teaches the use of a transistoras a switch in the grid circuit to effect fast beam-switching withminimal overshoot and distortion of the current pulse.

Preferably, the active variable conductance device is a transistor, forexample either a field effect transistor (FEI) or a bipolar transistor.

The active variable conductance device may alternatively comprise one ormore light dependent resistors.

The control means advantageously comprises fibre optics andelectro-optical devices, or any other optical link.

By using an active variable conductance device instead of a passiveresistor as in the prior art, control over the electron beam current isgreatly facilitated. Preferably, an optical link is used to control thevariable conductance device, thereby reducing the risk ofelectromagnetic interference.

In a preferred embodiment, a current detector for detecting the currentflow between the high voltage supply and the cathode filament isprovided, either between the output of the high voltage power supply andthe active variable conductance device or between the active variableconductance device and the cathode filament.

By measuring the current at this point, rather than at the ground end ofthe high voltage power source, discrimination between the truethermionic emission from the filament and all other forms of leakagecurrent becomes possible. Hence the true thermionic emission current canbe measured and controlled.

In accordance with a second aspect of the present invention, there isprovided an X-ray source comprising a Faraday shield, in whichelectrical circuitry is housed, a high voltage power supply and anisolating transformer, wherein the isolating transformer is coaxiallyshielded; the shielding forming a continuation of the Faraday shield.

The isolating transformer is preferably in electrical connection withboth an electron accelerating means and a cathode filament transformer,or other cathode filament supply means.

The first and second aspects of the invention are valuable individually,but a preferred embodiment comprises an X-ray source including bothaspects of the invention.

The invention further provides an X-ray source or apparatus includingany one or more of the novel features described or claimed herein.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described, by way ofexample only, and with reference to the accompanying schematic drawings,in which:

FIG. 1 shows a conventional X-ray source circuit arrangement;

FIG. 2 shows conventional cathode filament heating in an X-ray sourceincorporating a high voltage multiplier circuit and isolating heatertransformer;

FIG. 3 shows an X-ray source utilising negative feedback biasing;

FIG. 4 shows an embodiment of an X-ray source in accordance with oneexample of the first aspect of the present invention;

FIG. 5 shows a further embodiment of an X-ray source in accordance withanother example of the first aspect of the present invention;

FIG. 6 shows an embodiment of an X-ray source in accordance with oneexample of the second aspect of the present invention;

FIG. 7 shows a further embodiment of an X-ray source in accordance withanother example of the second aspect of the present invention; and

FIG. 8 shows a preferred embodiment of an X-ray source incorporatingexamples of both aspects of the invention.

In all of FIGS. 1 to 7, identical reference numbers are used throughoutto indicate similar components and features. In FIG. 8, however,features and components directly comparable with those in FIGS. 1 to 7are given reference numbers increased by 200 over those used in theearlier figures.

DETAILED DESCRIPTION

In the conventional X-ray source shown in FIG. 1, a cathode filament 30is connected to an isolated power supply 10. Encircling the cathodefilament 30, and connected to a high voltage supply 20, is a focusingcup 40. In operation, an electron beam 50 is accelerated through anannular anode 60 and focused onto a metal target 70 from which X-rays 80radiate. The power supply 10 typically comprises an isolating step-downtransformer (shown in FIG. 2 as 11), supplying around 6V to heat thecathode filament 30.

FIG. 2 shows a conventional X-ray source including a high voltagemultiplier circuit 90 connected to the focusing cup 40. Here, anisolating transformer 11 is shown connected to the cathode filament 30.The multiplier 90 is otherwise known as a Cockcroft-Walton voltagemultiplier 90. Most modem X-ray sources use this type of multiplier, thefunctioning of which is well known to persons skilled in the art.

Included in the conventional X-ray source shown in FIG. 3 is a variablefeedback resistor 120, which is connected between the cathode filament30 and the focusing cup 40. This configuration provides negative biasingto the focusing cup 40, thus ensuring that it remains at a negativepotential as compared to the potential of the cathode filament 30.Biasing is essential if the focusing cup is to provide space-chargecontrol of the electron beam current and is often alternatively providedby an isolated negative bias supply.

A problem arising from the X-ray source of FIG. 3 stems from thedifficulties associated with safely and precisely varying the value ofthe feedback resistor in order to maintain optimal control of the beamcurrent. An embodiment of an X-ray source in accordance with the firstaspect of the invention is shown in FIG. 4. Here, instead of a feedbackresistor, an active variable conductance device 130 is employed. Thisdevice maybe a field effect transistor (FET) for example. Alternatively,a light dependent resistor (LDR) controlled by an optical link to varythe conductance can be used. Indeed, the reader will be aware that thereare many other devices that may be suitable for the particularrequirements of an application.

In the X-ray source of FIG. 4, the variable conductance device 130 is abipolar transistor, controlled (by one of a variety of known methods) bya control circuit 140 in response to control signals 150. In the casewhere optical control is used, control signals 150 will be passed by oneof a choice of known optical links such as a conventional fibre opticcable and transduced by suitable electro-optical devices such aslight-emitting diodes (LEDs) and photodiodes. In this way it is possibleto provide precise dynamic and inertialess control of the electron beamcurrent.

In a further embodiment of an X-ray source according to the first aspectof the arrangement, as shown in FIG. 5, a current sensing circuit 160 isemployed to provide a measurable indication of the electron beamcurrent. This circuit can include an LED, the luminance of which isdirectly proportional to the amplified electron beam current. Thecircuit generates control signals 170 that are used in feedback controlof the variable conductance device 130, through control signals 150 andassociated control circuit 140. (This feedback loop is shownschematically by the broken line 155). In practice, other components maybe included in the feedback loop, and these components may includeground circuitry 156, so that signal 170 returns to ground and signal150 is transmitted from ground. The current sensing circuit 160 is shownbetween the high voltage supply and the active conductance device. Thiscurrent sensing circuit could instead be at a position indicated by160A, between the active conductance device 130 and the filament 30.

The advantage of the above embodiment is that, in measuring the currentflow at a point in the circuit shown in FIG. 5 by circuit 160 (oralternatively 160A), it is possible to differentiate accurately betweenthe thermionic current flow and the leakage current which, as describedearlier, can be influenced by many extraneous factors. Measured currentvalues can then be used in a feedback control loop via optic link 150 tofacilitate optimal adjustment of the biasing level. The currentsensitive circuit 160 may take many different forms, and may be opticalor electronic or otherwise. Many such means will be apparent to theskilled reader.

As discussed above, it is conventional to enclose all sensitivecircuitry and components in a Faraday shield. However, it is notnormally possible to completely electrically screen the components frompotentially damaging electromagnetic fields, since a break in theFaraday shield is necessary to allow access to the circuit for powerlines, control inputs etc.

Referring to FIGS. 6 and 7, a transformer primary winding 180 is coupledto a transformer secondary winding 190 via a transformer core 200. Thetransformer secondary winding 190 feeds power into circuitry within aFaraday shield 210.

In an embodiment of the second aspect of the invention, a toroidal metalsheath 193 surrounds the transformer secondary winding 190, and extendsas a tube 194 from the secondary circuit 190 towards the main Faradayshield 210. For practical shielding purposes, the toroidal sheath 193and tube 194 form an integral part of the Faraday shield 210. Tube 194serves as a conduit, screening wires 195 connecting (or continuing)winding 190 to circuitry within the Faraday shield. The toroidal sheathhas a discontinuity, or electrical break, 196, preventing it from actingas a shorted turn. The discontinuity is, however, such that totalshielding is still obtained.

FIG. 7 shows a variant of FIG. 6, in which the outer coaxial conductorforms part of the secondary winding; it connects to the secondarywinding at point 197. Thus, the outer conductor forms part of thewinding and its extension towards the Faraday shield.

It is to be noted that, in FIGS. 6 and 7, only one turn is shown for theprimary and secondary windings, for clarity. In practice, more than oneturn may be present for either or both of these.

Referring now to FIG. 8, there is shown a preferred embodiment of theinvention in which developed forms of both aspects of the invention areincorporated into an integrated high voltage generator and x-ray source.

The electron beam is produced by thermionic emission from a cathode 230,which is made from tungsten wire or other material typically formed intothe shape of a hairpin. In order for it to emit electrons, the cathodemust be heated to incandescence. The required cathode temperature isproduced by resistive self-heating. Electrons are extracted from thecathode 230 by means of an electric field applied, in known manner,between the cathode 230 and an anode (not shown in FIG. 8). As explainedpreviously, the arrangement is such that the anode is at groundpotential and the cathode is raised to a high negative potential. Themagnitude of the beam current is controlled by a “bias” voltage imposedonto an annular grid electrode or Wehnelt 240 that surrounds thecathode. The bias voltage is always negative with respect to thecathode. The bias voltage also serves to produce a focussing electricfield for the emitted electron beam, thereby controlling its diameterand ultimately the size of the x-ray source. The cathode 230 and theannular grid electrode 240 are, as is conventional, maintained invacuum; the vacuum wall being shown in part as 235 in FIG. 8.

The grid bias voltage is obtained by a technique, known as self-bias,which is commonly used on triode devices including, in particular,electron microscopes. The electron beam current passes through aresistor connected between the grid and the cathode and develops, acrossthe resistor, a voltage which constitutes the grid bias voltage. Thesystem is thus self-stabilising and a separate power supply for the gridvoltage is not required. The magnitude of the electron beam currentdepends on the size of the resistor and on physical characteristics ofthe gun which are geometry dependent.

In accordance with this embodiment, the resistor is replaced by a devicewhose resistance can be altered electronically. A preferred device is aField Effect Transistor (FET) 330, but the principle of operation couldalso be implemented using other devices, such as light dependentresistors.

The beam current flows in series through a resistor 325, the FET 330 anda resistor 335. A Zener diode 336 protects the FET 330 from excessivevoltage.

As discussed above, this arrangement differs significantly, in bothconcept and effect, from conventional circuit schemes, which typicallyemploy a separate DC supply for the grid voltage floating at cathodepotential, and which may utilise a series-regulating element for voltagecontrol and stabilisation.

In conventional x-ray generators, the beam current sensing is typicallyachieved by measuring the current flowing at the bottom of the diodecapacitor bank forming the high voltage multiplier (often called aCockroft-Walton multiplier). In the present system, such a high voltagemultiplier 290 is employed. A conventional sense resistor 300 is alsoshown. However, as described above, there is a serious disadvantage tousing the voltage on sense resistor 300 as the means of measuring andcontrolling the electron beam current; namely that the current flowingat this point may include extraneous components in addition to the trueelectron beam current. These extraneous currents often include currentsemitted from the vacuum facing surface of the housing surrounding thefilament. The locations producing such emission are known as coldcathode or field emission sites, and are well known to those skilled inthe art of the design of high voltage vacuum devices. Field emissionsites are unstable and can be neither predicted nor eliminated If thecontrol signal for beam current stabilisation is derived from a senseresistor 300 then the control of the true electron beam, that is emittedthermionically from the cathode 230, will be corrupted by theunquantifiable inclusion of extraneous currents from field emissionsites. This makes stable control at low operating beam currents and highcathode voltages very difficult and degrades x-ray image quality undersuch conditions. The present invention permits the true current flowingfrom the cathode to be measured. This allows very precise control of thebeam current even under usually difficult conditions, such as whenoperating at extreme high voltage with low beam currents, and even withfield emission sites present.

The true electron beam current is sensed as a voltage across resistor325 and is fed into an integrated circuit 361 configured as a voltage tofrequency converter. The frequency output of integrated circuit 361drives an LED 362, which sends a frequency modulated light signal 371down an optical fibre 355 a. At the other end of the fibre 355 a, theoptical signal is incident upon a photodiode 363. This converts theoptical signal back into an electrical signal which accuratelyrepresents the measured electron beam current and is applied, via abuffer amplifier 364, to circuitry (not shown) which interfaces in aknown manner with a computer. Computer commands input by a user of thesystem are used to effect adjustment of the electron beam current.However, if a computer is not used, appropriate circuitry is presentedat a location convenient for direct or remote manual adjustment by anoperator, thus allowing the beam current to be controlled, which may beeither in real time, or to predetermined values.

It is necessary to provide a feedback signal for precise closed-loopcontrol of the beam current against the predetermined demand levelselected by the operator. Advantageously, since the resistance of theFET 330 may be varied by adjusting its gate voltage, this isaccomplished by means of another photodiode 365 using optical signals351 generated by a second LED 366; these optical signals 351 beingamplitude modulated in a sense effective to indicate any desired changeof the beam current. The signals are delivered into a second opticalfibre 355 b, the output of which illuminates the photodiode 365.

Optical fibres are used to provide electrical isolation betweenelectronic circuits at the high and low voltage ends of the high voltagemultiplier 290.

The current sensed on resistor 300 is not used for control ormeasurement, but may be used by circuits designed to protect the highvoltage generator in the event of a fault causing excessively highcurrent in the multiplier 290.

Occasional electrical discharges can be expected to occur within thex-ray source. Such discharges lead to rapidly changing transientcurrents, and it is necessary to protect active electronic componentsfrom the potentially damaging effects of radiated and conductedelectromagnetic interference generated by these transients. Theelectronic circuits associated with the cathode and grid are containedin a metal walled chamber 410. The whole of this container is connectedto the grid and is therefore at a very high voltage with respect toground. This container provides very substantial screening for thesensitive circuits within it, and acts as a “Faraday shield”.

Although it does not need to be hermetically sealed, the container isconstructed in such a way that its openings are of minimal size. Theintegrity of such a Faraday shield may be compromised by the need tobring electrical signals in and out.

In this embodiment, the power for all of the circuits within the shieldis provided by a high voltage isolation transformer. The secondarywinding 390 of the transformer is insulated so as to provide therequired high voltage isolation, and is constructed as a co-axialsystem. The outer conducting member 393 of this co-axial arrangementforms a continuous extension of the main Faraday shield 410.Furthermore, only the outer conductor of the co-axial arrangement windsaround the transformer core 400.

The inner conductor 390 emerges from a hole in the side of the outerconductor and is then joined to the end of outer conductor 393. Thelength of inner conductor 390 and the size of the hole in the outerconductor 393 are kept very small. The co-axial self screeningconstruction of the secondary winding ensures that conducted andradiated signals into the Faraday shield are so small that thereliability of the sensitive components housed within can be guaranteed.

The core 400 of the isolating transformer lies outside the boundary ofthe Faraday shield 410; only the outer co-axial member 393 of thesecondary winding 390 is integrated into the continuum of the Faradayshield wall.

The Faraday shield may advantageously contain certain additionalelectronic circuits which might, for example, be used to monitor,control or stabilise the cathode filament voltage, current or power.Such circuitry, floating at high voltage, may also utilise fibre opticsas the means of conveying signals to other electronic circuits operatingnear to ground potential.

1. An X-ray source comprising: an X-ray emissive target; a high voltagepower source; a cathode filament and an anode electrode coupled to saidhigh voltage power source; said cathode filament and said anodeelectrode being adapted to establish, in response to a beam currentdrawn by said cathode filament from said power source, a beam ofelectrons directed at said target; a control grid electrode;self-biasing means for generating a bias voltage for application to saidcontrol grid electrode to control the magnitude of said electron beamand to produce a focussing electric field for the electron beam; theself-biasing means including an active variable conductance device andsensing means for sensing said beam current and generating an indicationof the magnitude thereof; means for conveying said indication to aremote location; means at said remote location for monitoring said beamcurrent and determining adjustment required thereto; and control meansfor generating a control signal indicative of said adjustment and forapplying said control signal to said active variable conductance deviceto control its conductance to vary the beam current, and thereby themagnitude of said electron beam, in accordance with said adjustment. 2.An X-ray source as claimed in claim 1, wherein the active variableconductance device is a transistor.
 3. An X-ray source as claimed inclaim 2, wherein the transistor is a field effect transistor or abipolar transistor.
 4. An X-ray source as claimed in claim 1, whereinthe active variable conductance device comprises one or more lightdependent resistors.
 5. An X-ray source as claimed in any one or more ofclaims 1 to 4, wherein the control means comprises optical means.
 6. AnX-ray source as claimed in claim 5, wherein the optical means comprisesfibre optics to pass optical signals and electro-optical devices fortransducing optical signals into electrical signals and vice versa. 7.An X-ray source as claimed in claim 5, wherein said control meanscomprises means capable of generating a frequency-modulated opticalsignal indicative of the said beam current, means for conveying saidfrequency modulated optical signal to said remote location and means atsaid remote location for converting said optical signal into anelectrical signal capable of being influenced by user input.
 8. An X-raysource as claimed in claim 7, wherein a computer is provided at saidremote location, capable of manipulation by the user to influence saidbeam current.
 9. An X-ray source as claimed in claim 7, furthercomprising feedback means to transfer a control signal from saidlocation to said active variable conductance device.
 10. An X-ray sourceas claimed in claim 9, wherein said feedback means comprises opticalmeans and said control signal comprises an amplitude-modulated lightsignal.
 11. An X-ray source as claimed in claim 1, wherein a currentdetector for detecting current flow between the high voltage powersupply and the cathode filament is provided between an output of thehigh voltage power source and the active variable conductance device, orbetween the active variable conductance device and the cathode filament.12. An X-ray source as claimed in claim 11, wherein an output of thecurrent detector is applied directly or indirectly to the control means.13. An X-ray source as claimed in claim 1, further comprising a Faradayshield, in which electrical circuitry is housed, and an isolatingtransformer, wherein an isolating transformer winding is coaxiallyshielded, the coaxial shield forming a continuation of the Faradayshield.
 14. An X-ray source as claimed in claim 13, wherein saidisolating transformer winding comprises a secondary winding to which aprimary winding of said transformer is coupled via a transformer core;the transformer secondary winding being arranged to feed power intocircuitry within said Faraday shield.
 15. An X-ray source as claimed inclaim 14, wherein the coaxial shield is electrically connected to awinding.
 16. An X-ray source comprising: an X-ray emissive target; ahigh voltage power source; a cathode filament and an anode electrodecoupled to said high voltage power source; said cathode filament andsaid anode electrode being adapted to establish, in response to a beamcurrent drawn by said cathode filament from said power source, a beam ofelectrons directed at said target; a control grid electrode;self-biasing means for generating a bias voltage for application to saidcontrol grid electrode to control the magnitude of said electron beamand to produce a focussing electric field for the electron beam, theself-biasing means including an active variable conductance device andsensing means for sensing said beam current and generating an indicationof the magnitude thereof; means for conveying said indication to aremote location; means at said remote location for monitoring said beamcurrent and determining adjustment required thereto; control means forgenerating a control signal indicative of said adjustment and forapplying said control signal to said active variable conductance deviceto control its conductance to vary the beam current, and thereby themagnitude of said electron beam, in accordance with said adjustment; anda Faraday shield, in which electrical circuitry is housed, and anisolating transformer, wherein an isolating transformer winding iscoaxially shielded, the coaxial shield forming a continuation of theFaraday shield, wherein said isolating transformer winding comprises asecondary winding to which a primary winding of said transformer iscoupled via a transformer core; the transformer secondary winding beingarranged to feed power into circuitry within said Faraday shield, andwherein said coaxial shield comprises a toroidal metal sheathsurrounding the transformer secondary winding and extending as a tubefrom the secondary winding towards the Faraday shield; the toroidalsheath being formed with a discontinuity preventing it from acting as ashorted turn.
 17. An X-ray source comprising: an X-ray emissive target;a high voltage power source; a cathode filament and an anode electrodecoupled to said high voltage power source; said cathode filament andsaid anode electrode being adapted to establish, in response to a beamcurrent drawn by said cathode filament from said power source, a beam ofelectrons directed at said target; a control grid electrode;self-biasing means for generating a bias voltage for application to saidcontrol grid electrode to control the magnitude of said electron beamand to produce a focussing electric field for the electron beam, theself-biasing means including an active variable conductance device andsensing means for sensing said beam current and generating an indicationof the magnitude thereof; means for conveying said indication to aremote location; means at said remote location for monitoring said beamcurrent and determining adjustment required thereto; control means forgenerating a control signal indicative of said adjustment and forapplying said control signal to said active variable conductance deviceto control its conductance to vary the beam current, and thereby themagnitude of said electron beam, in accordance with said adjustment; anda Faraday shield, in which electrical circuitry is housed, and anisolating transformer, wherein an isolating transformer winding iscoaxially shielded. the coaxial shield forming a continuation of theFaraday shield, wherein said isolating transformer winding comprises asecondary winding to which a primary winding of said transformer iscoupled via a transformer core; the transformer secondary winding beingarranged to feed power into circuitry within said Faraday shield, andwherein an outer conductor of the coaxial shield is connected to asecondary winding and thereby forms part of the secondary winding. 18.An X-ray source having an X-ray emissive target, a cathode filament, andan anode electrode comprising: a Faraday shield, in which electricalcircuitry is housed, a high voltage power supply, and an isolatingtransformer, wherein an isolating transformer winding is coaxiallyshielded, the coaxial shield forming a continuation of the Faradayshield, wherein said isolating transformer winding comprises a secondarywinding to which a primary winding of said transformer is coupled via atransformer core; the transformer secondary winding being arranged tofeed power into circuitry within said Faraday shield, and wherein saidcoaxial shield comprises a toroidal metal sheath surrounding thetransformer secondary winding and extending as a tube from the secondarywinding towards the Faraday shield; the toroidal sheath being formedwith a discontinuity preventing it from acting as a shorted turn. 19.X-ray apparatus, comprising an X-ray source as claimed in claim 1 orclaim
 18. 20. An X-ray source having an X-ray emissive target, a cathodefilament, and an anode electrode comprising: a Faraday shield, in whichelectrical circuitry is housed, a high voltage power supply, and anisolating transformer, wherein an isolating transformer winding iscoaxially shielded, the coaxial shield forming a continuation of theFaraday shield, wherein an outer conductor of the coaxial shield isconnected to a secondary winding at a point and thereby forms part ofthe secondary winding.